(CNN)Now that Republicans have their nominee, the party elites have a big choice to make. With Donald Trump winning by sizable majorities in the last few rounds of primaries there will be growing pressure for Republican leaders to back the ticket. Some are indicating that they won't give into this pressure. "Republicans for Clinton" have started to express their opinion via social media.

In a fascinating exchange Thursday, Speaker Paul Ryan told CNN's Jake Tapper that he wasn't ready to back Trump's nomination, that he expected the nominee to unify the GOP and conservatives and looked to him to run a campaign that Americans will be "proud to support and proud to be a part of."

Trump issued a statement saying that, "I am not ready to support Speaker Ryan's agenda. Perhaps in the future we can work together and come to an agreement about what is best for the American people. They have been treated so badly for so long that it is about time for politicians to put them first!"

Refusing to support Trump won't be easy for GOP leaders. Given the mood of the electorate, many Republicans will now be feeling immense pressure to support the ticket, by either formally endorsing Trump, as New Jersey Governor Chris Christie did early on, or by announcing like Louisiana Gov. Bobby Jindal that he intended to vote for the nominee -- short of an endorsement but still a clear yes. And in the past two days, Mitch McConnell and Rick Perry have said they would support Trump.

The truth is that all of the major Republicans have some pretty strong personal incentives to join the fold. Even if Trump represents many things they dislike and says things that they don't feel comfortable hearing in the political arena, over the coming few weeks they will be feeling more and more pressure for a variety of reasons to announce their support for him.

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What might bring some of the major Republicans in the party to reach this conclusion?

Jeb Bush

While on paper Jeb Bush would be the last person to endorse the person who derailed his "inevitable" candidacy and had some blistering comments about him during the primaries, it wouldn't be a total surprise if the former Florida governor changed his tune.

Jeb Bush

After all, Bush will now want to play the role of statesman within the GOP. While there are not many donors and party elites who would have the stomach to see him run again given his disastrous performance, there is room for him to emerge as a leader in the process of rebuilding the party in the aftermath of Trump. To do that, however, he can't disassociate himself from the party and be part of a defection that helps a Democrat seize control of the White House. (His brother, Former President George W. Bush, has said he won't comment on or participate in the campaign.)

Marco Rubio

Rubio faces a difficult road ahead. Unlike Bush, Rubio clearly has ambitions to run again and at the same time that he realizes he might be a vice presidential pick. The cost of Rubio, who has fashioned himself as the one Republican who could actually reach out to Latino voters, of joining forces with Trump could be enormously high. If Trump crashes and burns in the general election, many voters in 2020 won't forget that he became a supporter.

Sen. Marco Rubio

At the same time, Rubio is enormously valuable to Trump, and the Florida senator knows that. He offers Trump a way to prove that he is not "anti-immigrant." It also allows him to be part of a ticket that could theoretically bring younger voters to the campaign. These temptations will be huge. Rubio might make the calculations that the risk of being part of this ticket will be forgotten in four years (as was the case with Paul Ryan) and the chances of his gaining a position of high stature will be large.

John Kasich

In many respects, John Kasich, also a potential vice presidential pick, has the most to lose from supporting Trump. The Ohio governor has defined his entire campaign as being the anti-Trump candidate. He has been extremely critical of his opponent's political style and warned about the damage that this campaign could do in the long-term to his beloved party.

Ohio Gov. John Kasich

Yet Kasich has not always been opposed to joining forces with radical elements in the political process. During the 1980s and 1990s he worked with congressional Republicans like Newt Gingrich, whose aggressive tactics turned off many senior party leaders. He proved he had the will to join forces with rightward elements in order to advance his standing and that of the party in a tough political arena controlled by the Democrats. Like Rubio, Kasich understands that he will have enormous value to Trump, who will be looking for appointments that comfort skeptics who don't think he's up to the job of governance.

Ted Cruz

Sen. Ted Cruz

While there is no love lost between Cruz and Trump after an incredibly nasty and contentious political battle, the Texas senator wants to make certain that voters continue to see him in the role of most prominent conservative Republican in the game.

Cruz is someone who comes from the far right. He has associated with individuals who are as controversial as anyone in Trump's orbit and he has proven to be someone willing to use radical methods to achieve his goals.

The last thing Cruz would want, as he keeps an eye toward 2020 and becoming a party leader, is to be accused of helping Democrats win the White House and Senate. For this reason, even if there isn't any prospect of receiving an appointment to a prominent position by Trump and even if he doesn't want one, Cruz is going to think seriously about standing behind the man he so dislikes.

John McCain

Sometimes in American politics local issues and electoral concerns can drive decisions that are made at the national level. Senator McCain most likely is genuine when he expresses deep concerns about Donald Trump. He doesn't think Trump is good for the party, nor does he think Trump is good for American politics.

Sen. John McCain

But the senator has a problem on his hands. He is in the middle of a tough re-election battle against Democratic Rep. Anne Kirkpatrick, and Trump proved to be enormously popular in McCain's home state of Arizona.

Failing to endorse his party's frontrunner could be the exact kind of tipping point that brings an end to his career in the upper chamber. McCain is not ready to leave yet and he believes his party needs him to push back against the Tea Party Republicans who rule the roost. For this reason, McCain has good reason to embrace Trump.

Nikki Haley

South Carolina Gov. Nikki Haley

South Carolina Gov. Nikki Haley endorsed Rubio in the primaries and has tried to position herself as a different face for the Republican Party -- a governor who agreed that the Confederate flag must go and who as an Indian American offers a new voice for the lily-white GOP. The pressure on her to support Trump will be strong. Lest anyone forget, Haley is a product of the Tea Party.

She is a part of and sympathetic to Republicans who play on the far right of mainstream American politics and is more than comfortable offering her support for ideas that are controversial outside of her conservative constituency, including her defense of the Confederate flag until a horrendous shooting changed her mind.

Not only does Haley realize her value for a Trump administration, but she believes herself to be a key player and possible presidential contender in years to come. Loyalty to party will be essential for her to prove.

Paul Ryan

House Speaker Paul Ryan

House Speaker Ryan has tried to remake himself from a very conservative young maverick in the GOP to the only potential coalition builder within the party. Though he started his career sounding like Barry Goldwater, in recent months he has tried to act more like 1980s-era Ronald Reagan.

Of every leader in the GOP, his name had come up more frequently as the person who could save the day as a last-minute candidate in a brokered convention. But that's not going to happen, and Ryan has a strong interest in fighting for the Republican ticket.

He does not want to be part of an open civil war that causes the GOP to lose control of the White House as well as Congress. Nor does he want to be in a position where he has burned his bridges with an incoming Trump White House at the exact moment united government could become a battering ram against liberalism. And for all his talk of moderation, the truth is that Ryan has been more than comfortable at working with Freedom Caucus Republicans who share some of Trump's more controversial views on issues like immigration.

In the early days, Ryan has caused a big stir by announcing he is not yet ready to endorse Trump. This set off a frenzy about intra-partisan tensions and the future of the party. But importantly he left himself an out, and leverage with Trump by saying not yet, rather than never. The pressure for him to eventually join the party will remain strong.

Mitch McConnell

The leader of the Senate Republicans doesn't like to say much in public. But one thing is clear—the focus of his ire centers on the Democratic Party, not any of his colleagues in the GOP. McConnell has been ruthless in using the mechanisms of obstruction to block President Obama at every turn.

McConnell is a party man and he understands that his power derives from the success of his party. Failing to endorse Trump would only fuel an internal party civil war that undermines the ability of the GOP to keep control of the upper chamber. For that reason, expect him to be Trumped.

While there will be lots of rumbling among Republican leaders about their concerns over Donald Trump, don't be surprised if many of these leaders start falling in line over the next few weeks. Maybe they will be holding their noses, maybe they will be squirming, but they might very well be saying yes to Trump.

The interesting question is whether Republican voters will do the same. There are many conservatives out there in the electorate who might not be comfortable voting for a candidate with Trump's background or who could do so much damage to the image of their party.

Some might sit out and some, like John McCain's former advisor Mark Salter, might say that they're with Hillary. If so, some Republican leaders may be in the ironic spot of supporting Donald Trump, only to find themselves once again disconnected from voters.